TRURO — The evidence: Eyewitness accounts of a dorsal fin circling the victim. Hordes of seals. Major ­injuries.

The conclusion: The man bitten while swimming off Ballston Beach Monday was probably the victim of a great white shark attack, the first confirmed bite in Massachusetts since 1936, the state’s top shark scientist said Tuesday.

“There’s very few sharks that would administer such a bite,” said Greg Skomal, a senior scientist with the Massachusetts Division of Marine Fisheries.

Shark sightings have become increasingly frequent, caused in part by the resurgence of local seal populations. Skomal said the state is currently tracking nine great white sharks, measuring between 9 and 18 feet, but they are extremely difficult to track because of their great range and propensity to stay in deep water.

“They’re very dynamic, they move great distances, and we don’t know they’re there unless they come into shallow waters,” he said.

Chris Myers, Monday’s shark-bite victim, had been body surfing with his son about a football field’s distance from the shore.

Beachgoers said they saw a dorsal fin appear close to the body-boarders, said Officer Scott Holway of the Truro ­police. It circled once, twice, then sank beneath the surface.

A moment later, Myers was jerked downward, Holway said.

He resurfaced, and Myers and his son screamed for help. Seth Blaustein, 25, was one of several men who jumped into the water. He thought the two men were stuck in a rip current, he recalled Tuesday.

Boogie-board in hand, Blaustein swam to Myers’s teenage son, who looked to be the more fatigued of the two, and towed him to shore. Myers paddled feverishly toward the sand without assistance.

Only when Myers climbed onto the beach did Blaustein see blood streaming from the man’s legs.

“I didn’t think about it ­[being a shark attack] until I was halfway out there,” Blaustein said. “If I’d have seen the shark, I don’t know if I’d have gone.”

Blaustein’s cousin Cassie O’Connor, a nurse at Beth Israel Deaconess Medical Center, helped pack towels onto Myers’ wounds. He had deep lacerations on both legs and the back of one of his calves was split open, she said.

Holway said Myers was alert and calm as he lay on the beach. He told police he had kicked the creature that pulled him down. He was rushed first to Cape Cod Hospital and then to Massachusetts General Hospital for surgery.

“People in town are saying that it could have been a seal,” Holway said. “There’s no way that a seal could bite someone like that or pull him under like that.”

Myers remains at Massachusetts General Hospital and has instructed the hospital not to release any further information on his condition to news organizations, an MGH spokesman said Tuesday.

The last attack by a great white shark was on July 25, 1936. Joseph Troy, 16, of Dorchester was swimming with a friend in Buzzards Bay about 150 yards from shore when a 6-foot creature seized Troy by the leg and dragged him under the water, newspapers reported. He resurfaced, unconscious, in a pool of blood, and died later that evening.

On Tuesday, Ballston Beach was open to swimmers, though signs posted on the sand warned of recent shark sightings. Seals were spotted bobbing in the water.

Cape Cod Times

The victim was loaded on to an ambulance at Ballston Beach in Truro Monday after what officials say was a shark attack.

A few people flirted with wetting their feet in the surf, but most did not venture further.

Blaustein and his family stayed put on the sand.

“He was very far out there in the water,” said Blaustein, sitting on a beach chair a few feet from where Myers had come ashore Monday.

“But I’m probably not going to go in [the ocean] because the thought is so fresh in my mind.”

“I swim every day I’m here,” said Christine Blaustein, Seth’s mother. “But I’m not going in the water today.”

Longtime vacationers in this town — such as Carol Pesiri, 68 — said they have seen more seals, but had hoped that sharks would remain close to the Cape’s elbow, where they have been frequently spotted at Chatham and Orleans.

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